This invention relates generally to the field of treating waste water, particularly waste water containing fats and fatty acids, such as that produced in meat-packing plants. In particular, the invention includes both methods and compositions for the improved separation and purification of fat-containing waste waters.
Water is used in large amounts in meat-packing plants both to clean the facility and in the actual processing of animal or poultry carcasses. For example, water is used in the chillers to rapidly cool the carcasses from body temperature to an intermediate temperature prior to refrigeration or freezing. The chilling preserves the meat and inhibits the growth of bacteria. The water used in both cleaning and chilling operations carries fat and fatty acids and is laden with those materials after the water has served its purpose. It also contains solid materials and microbiological constituents that result in a high biological oxygen demand ("BOD") and suspended solids in the water.
Typically, the waste water utilized in various parts of a meat-packing facility is unsuitable for direct discharge into a stream. Treatment by municipal water facilities is expensive.
Since the waste water contains some useable materials from which additional byproducts and revenue can be obtained, it is preferable to recover these materials as economically as possible before sending the effluent to municipal water treatment facilities. To that end, the fat-containing waste waters from various portions of the plant typically are combined and subjected to mechanical separation in holding tanks, clarifiers or dispersed air flotation ("DAF") equipment to produce three distinct materials: (1) inedible fat concentrate, (2) water and (3) solids.
The inedible fat concentrate is typically cooked to produce meal and tallow. It may be blended either before or after cooking with higher grade fats to produce a range of tallow products suitable for both pet food and soap. These by-products of the meat-packing process provide a not insignificant amount of revenue to meat packers. The tallow retrieved from the waste water is generally of poor quality and represents a small percentage of the total inedible tallow from a typical meat-packing plant. Although the proportion is small; it influences the quality of the entire inedible tallow with which it is blended.
The quality and value of the tallow are generally judged by its color and free fatty acid ("FFA") content. These criteria are not independent. A dark color generally means a high free fatty acid content. It is because of this relationship that the determination of the free fatty acid content is used to evaluate the inedible tallow. Under normal operating conditions the free fatty acid content of the tallow can vary between 3 and 85 weight percent. A free fatty acid content higher than about 4 percent is generally undesirable, particularly for soap stock, known in the trade as "bleachable fancy."
The sludge in the bottom layer from the separation is usually pumped or augured out of the equipment, then treated if necessary prior to land application.
The quality of the water recovered from the fat-containing waste water stream is generally measured by its BOD and the total suspended solids ("TSS"), which is usually expressed in weight per unit volume. The measured BOD and TSS values are used to determine the acceptability of the waste water for discharge and the fees associated with that discharge. Whether the water is discharged into a natural stream or sewer, the fees for discharging the effluent are determined, among other things, upon the discharged water's quality. The greater the BOD or TSS values, the greater the fee to discharge the water. Depending on the municipality involved, other qualities of the water or limitations may impact the fees for disposing it. These include the temperature, chemical oxygen demand ("COD"), or ammonia content of the water, generally measured as total Keldahl nitrogen ("TKN").
Although purification of the water at the meat-packing plant decreases the fees associated with its ultimate discharge, it is not possible to totally purify fat-containing waste water streams at a manufacturing facility. Improvements in the treatment of fat-containing waste water which can enhance the quality of both the fat containing byproducts and the effluent, increase the value of the former and decrease the cost associated with discharge of the latter. Thus, compositions and processes of the present invention are quite valuable.